Resistor for electric heating



Mar. 3, 1925.

C. W. SPEIRS RESISTOR FOR ELECTRIC HEATING Filed Nov. 26, 1925 E 111': I L' TJ 1:111: 11;:

Inventor.

Patented Mar. 3, 1925.

UNITED STATES CHARLES WILLIAM SI'EIRS, OF LONDON, ENGLAND,

AssIcNoR 'ro THE MORGAN CRUCIBLE COMPANY LIMITED, OF LONDON, ENG/IJAND- RESISTOR FOR ELECTRIC HEATING.

Application filed November 26, 1923. Serial No. 677,025.

To all whom it may concern:

Be it known that I, CHARLES WILLIAM S ams, a subject of the Kin of Great Britain, residing at Battersea orks, Church Road, London, England, have invented new and useful Improvements in Resistors for Electric Heating, of which the following is a specification.

This invention relates to resistors for the conversion of electrical energy into heat and designed to have a temperature range from atmospheric to over 1,400 C. and to be used for domestic, metallurgical and general manufacturing processes.

Electric resistors as heretofore usedhave been chiefly of either the wire resistance or carbon and graphite type, each of which types possesses disadvantages in ractice, the former owing to cost of manu acture, difficulty of mounting, limitation of temperature and unequal temperature of distribution, and the latter by reason of high conductivity and cost, the difliculty of arranging for high b voltage circuits, the tendency to rapid oxidation and the difliculty in arranging suitable mounting to prevent oxidation.

The object of the invention is to provide a cheap and simple resistor which can be manufactured in almost any shape or form required, and which, in many cases, can form part of the vessel or plant to be heated, and can attain practically the same temperature at all points of its surface. Furthermore, in the case of domestic articles, they can be immersed in water when not in use, for the purpose of cleaning.

To this end according to the invention a resistor is made as a solid continuous coherent ceramic body, in which the conductin portion is obtained by the addition of fla e graphite or other powdered carbon to the ceramic body in the plastic state andis so arranged that an electric circuit is provided which is of the correct resistance to give the required temperature (at all points over its surfaces) at any given voltage and is entirely surrounded, supported and insulated by the nonconductive ceramic ware,

preferably glazed on its surface.

In carrying out the invention the conducting portion of the resistor (hereinafter reconductor this and arranged to supply heat exactly where d ferred to as the conductor) may be made from a mixture of ball clay and a percentage of, preferably, flake graphite, but any other carbon powder can be used, varying according to the resistance required, from, say 20% to 80%. When the percentage of graphite is low, it may be advisable to add a quantity of burnt clay grog.

This conductor is moulded, when lastic, into the desired shape, and, while still plastic, is covered entirely with a coating of refractory clay, stoneware or porcelain, also in a plastic state; (this being necessary to support the conductor, both While the neces-v sary circuits are being formed and to support, insulate and protect it in use), so that. the whole, after burning, forms one continuous coherent ceramic body in which the r sistor is supported and insulated.

his body may be coated with a glaze when in the plastic state, or glazed after iscuiting by methods well known in the trade, the object of the to protect the resistor rom moisture and air. For high temperatures a further protection is aflorded by introducing beneath the glaze a layer of material similar to the additional layer serving to slay oxidation.

The resistance of the conductor can be varied as required by altering the percentage of carbon and/or varying the cross section and length of the circuit.

Care should be taken to make the con ductor so that equal lengths have equal resistances, whereby a uniform temperature is produced over the entire surface.

Care should also be taken to select suitable insulating and su porting bodies, so that their oo-eificient 0 expansion is approximately the same as that of the conductor and it. has been found, fol-21 example, that the same clay'body can be used for the entire article if partof the clay grog in the insulating body be replaced by flake graphite for the conductor. It is to be understood, however, that the invention is not restricted .to this combination as many others can be employed with the same, or approximately the same results.

almost any good quality glaze being further To prevent undue of the in the kiln, it is advisable to enclose it ina muflle before subjecting it to the burning process.

The conductor can be made up in one or more layers, each layer being separated from the next byan insulation layer. Furthermore, any clay vessel can be moulded on or around a suitable resistor. For instance, a casserole can be made with a resistor in the bottom, or a jug with the resistor in the bottom, or round the sides,-or both.

To illustrate the practical application of the invention the following description by reference to the accompanying drawing, gives a suitable method of manufacturing a simple circular heating plate suitable for working on'a circuit, say, of 110 volts and giving an even temperature of about 100 C. at any part of its surface.

In the said drawing- 1Figure 1 is a side view of the heating ate.

P gigure 2 is a transverse section thereof, an

Figure 3 is a sectional plan view of the plate.

A plastic insulating clay is made from 40%ball clay and 60% good burnt clay grog free from iron and impurities. A suitable grog may have the following percentage composition S10 72% A1 0 22% Fe O 2% Ti0 2% Alkalies 2% any desired circuit is obtained.

The conductor is made from out the same clay and grog with a good flake plumbago of about 85% carbon ground through a720 mesh sieve and mixed in the following proportions Clay I Clay grog", 40% Plumbago 20% A slab a of the insulating clay is rolled out on a fiat surface to a thickness, say .2

of an inchf A: slab b of the conductingmixture is then rolled, out to the required thickness, varying according to the resistance required, say, .1 of an inch, and is placed above and pressed on to the insulating slab a, a wash of clay slip made from the ball clay being used to ensure a good firm oint.

The combined slab thus obtained is then cut to the shape required, in this casecircular, and by means of a template, narrow grooves 0, about one eighth of an inch wide, are cut through the conductor layer so that These grooves care filled with plastic insulating clay, as' shewn, and the resistor is coated with another sheet (1 of, say, .2 of an inch plate is sufficiently high to damage the metal terminals, graphite rods are used to connect the said plate to the said terminals so that the latter are placed outside the zone of high temperature.

A clay dish can be moulded above a plate so produced, if required.

A cylindrical vessel can also be made as follows :A plastic mould is lined with the required thickness of insulating clay by means of a former on a jolly wheel. layer of the plastic conductor is then worked on to the clay and a definite thickness 'ven to the body by adjusting the former. g circuit is then cut in the conductor layer as explained above,'and the grooves filled in with clay, a further coating of insulating cla being worked on with the former.

ft is to be clearly understood that the same objects can be obtained by other methods of manufacture and we do not claim these methods only.

Claims: I

1. A resistor for electric heating, comprising a solid continuous ceramic body in which the conducting portion is obtained by the admixture with the ceramic material of powdered carbon.

2. A resistor as claimed in claim 1, comprising a series of layers of ceramic .material alternating with a series of layers of conducting material formed from ceramic.

material admixed with powdered carbon, the whole constituting a solid continuous coherent bod 3. A- resistor'for electric heating comprising a layer of ceramic material, and a superposed layer of ceramic material admixed with powdered carbon, such as flake plumbago, forming a conductor, the conductor layer being cut through along a line or lines to provide a continuous conducting section to the ends of which terminals are aflixed and the grooves formed, being filled with the ceramic material, the whole constituting a solid continuous coherent body.

1'. The method of producing resistors for electric heating, consisting in moulding a mixture of ceramic material and powdered too carbon, such as flake graphite, and in oovsubjected to a burning operation to form a continuous coherent body in which the conduetor is insulated.

5. The method of manufacturing resistors for electric heating, consisting in moulding a mass of ceramic material to the desired shape, superposing thereon a layer of ceramic material admixed with powdered carbon, such as flake aphite cutting or grooving the superpose layer to form the con- 10 ducting portion of the resistor, applying a further layer of ceramic material to the conducting layer and finally burning the whole, after glazing, if desired, to form a continuous solid body or mass.

CHARLES WILLIAM SPEIRS. 

